In a trend toward reduction of size and weight of portable electronic apparatuses of higher performance, keen demand has arisen for secondary batteries having high energy density and high capacity. Under such circumstances, lithium secondary batteries (e.g., lithium ion batteries and lithium polymer batteries) employing a non-aqueous electrolyte find a variety of uses, such as secondary batteries for use in a small portable electronic apparatus such as a mobile phone or a video camera, by virtue of high-energy-density, high-voltage characteristics. In such a lithium secondary battery, a positive electrode is formed of a metal oxide compound material (e.g., lithium cobaltate) that attains large charge/discharge capacity per unit mass at high voltage, while a negative electrode is formed of a carbon material (e.g., graphite) that exhibits a base voltage nearly equal to that of lithium and large charge/discharge capacity per unit mass. The charge/discharge capacity (per mass) values of these electrodes have been already improved to levels nearly equal to theoretical values, and further improvement in energy density per mass of a battery employing the electrodes is virtually limited. Therefore, extensive efforts have been made to develop novel high-capacity positive electrode materials such as iron olivine compounds and metal sulfides, as well as novel high-capacity negative electrode materials such as a composite material of carbon material with tin oxide, silicon oxide, lithium alloy, or lithium nitride.
Secondary batteries for use in small portable electronic apparatuses are required to be miniaturized. That is, energy density per mass and energy density per volume must be increased. For this purpose, one studied approach includes increasing electrode density so as to elevate the amount of material charged in a battery casing, thereby elevating energy density per volume of an electrode or a battery.
For example, lithium cobaltate-based oxide, a typical positive electrode material, has a true density of about 5.1 g/cm3, and an existing electrode employing lithium cobaltate has an electrode density less than 3.3 g/cm3. Thus, studies have been carried out for enhancing the electrode density to 3.5 g/cm3 or higher.
However, when the electrode density is elevated, interspaces in the electrode decrease. In this case, the amount of an electrolyte, which is generally present in the interspaces and plays an important role in electrode reaction, decreases, and permeation of the electrolyte into the electrode requires a longer time. Both cases are problematic. Shortage of electrolyte in the electrode induces retardation of electrode reaction, thereby problematically lowering energy density or high-speed charge/discharge performance. In addition, retardation of permeation of electrolyte results in a longer time for battery production, thereby increasing production cost. When a highly viscous polymer electrolyte is used (e.g., in a lithium polymer battery), production time is detrimentally prolonged.
In order to solve the aforementioned problems, an attempt has been made to reduce the amount of the carbon-based conductivity enhancer added to the positive electrode to as small a level as possible, to thereby increase the mass of active substance in the positive electrode and increase energy density. Furthermore, by reducing the amount of bulky carbon-based conductivity enhancer, electrode density per se can be elevated. However, when the conductivity enhancer is reduced excessively, conductivity of the electrode decreases and charge/discharge processes are disturbed, since a metal oxide compound such as lithium cobaltate-based oxide generally exhibits semi-conductivity. Therefore, conventionally, a carbon-based conductivity enhancer such as carbon black has been used in an amount of about 3 mass % or more.